GEOLOGY 



with London Clay also ; then, at a little greater distance from the main 

 mass, there is an outlier between Bennington and Watton, followed by a 

 larger outlier on which Datchworth is situated, and a smaller at Ayot, 

 these two being in the general direction, and the three having London 

 Clay over the Reading Beds. All these are in the river-basin of the 

 Lea, and their united area is about twelve square miles. In the Colne 

 river-basin there is first an outlier at St. Peter's, St. Albans; there are 

 three small outliers near together at Leverstock Green, Bedmond, and 

 Abbot's Langley, the first of these being beyond the general line ; three 

 small outliers near Sarratt follow ; and there is a small one near Chorley 

 Wood, Rickmansworth. Most of the outliers in the Colne river-basin 

 are of the Reading Beds only, and their united area is about three 

 square miles. 



For some distance this string of outliers roughly coincides with a 

 tolerably well-marked ridge of hills stretching from Watton south-west- 

 ward by Welwyn, Sandridge, and St. Albans, where it exceeds 400 feet 

 in height, to Hemel Hempstead. This ridge probably indicates a line 

 of flexure in the Chalk, which, while dipping elsewhere in a regular 

 manner from the Chiltern Hills towards London, is slightly depressed 

 along this line. The Eocene beds upon it may have thus been let down 

 below the plane of denudation, allowing patches of them to be preserved. 

 Their clays being better able to resist subsequent sub-aerial denudation 

 than the surrounding chalk, which also is constantly being chemically 

 dissolved, by the gradual wearing down of the surface of the Chalk they 

 would in course of time be left as hills. 



These outliers completely change the character of the soil overlying 

 the Chalk district. Some appear as well-wooded eminences on which the 

 oak and elm flourish best ; others, chiefly where the sands of the Reading 

 Beds are more developed than their clays, or where the London Clay 

 upon them is capped by pebble-gravel, are sandy, gorse-covered com- 

 mons. Nearly all are worked for brick-making. 



Far away to the north-west there are three very small outliers of the 

 Reading Beds, of three or four acres each, following each other in a line 

 from near Kensworth to Berkhamsted Common, the last of these only 

 being in Hertfordshire. The presence of these outliers is important as 

 showing the former great extent of the Eocene Beds, of which they 

 furnish more conclusive evidence than do the boulders of Hertfordshire 

 conglomerate which are found at even a greater distance from their 

 parent bed. 



To another line of flexure an uprise of the Chalk the existence 

 of a series of inliers in the London-Clay area is probably due. Inliers 

 are patches of lower beds exposed by the removal of the higher strata 

 which once covered them. In Hertfordshire the Reading Beds are thus 

 exposed beneath the London Clay in two inliers between Cough's Oak 

 and Northaw, and if this presumed line of flexure be continued parallel 

 with the outcrop of the Reading Beds into Middlesex, an inlier will be 

 met with extending from Pinner, past Ruislip, to just beyond Ickenham. 



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